These days, gallium nitride (GaN) is drawing attention as a semiconductor material that forms a blue light emitting diode, a semiconductor laser, a high voltage, high frequency power source IC, etc.
At present, gallium nitride used as a semiconductor material is produced as a single-crystal wafer of approximately 2 to 4 inches by vapor phase growth methods such as hydride vapor phase epitaxy (HYPE) and metal organic chemical vapor deposition (MOCVD). However, in the gallium nitride produced by such vapor phase growth, there are a large number of crystal defects, and it is therefore difficult to obtain the characteristics of the objective when it is incorporated in a device. Hence, reducing the amount of crystal defects has been a great issue in the production of gallium nitride crystals.
As a method for reducing the amount of crystal defects, there is a method in which a gallium nitride crystal is grown in a liquid phase, for example. However, in this method, it is necessary to dissolve nitrogen gas in a gallium molten liquid at a high temperature of 1500° C. or more and a very high pressure of 10,000 atmospheres or more in order to grow a gallium nitride crystal; hence, reaction equipment that withstands high temperature and high pressure conditions is needed, and industrial applications have yet to be achieved.
To ease the high temperature and high pressure conditions mentioned above, a method for synthesizing a gallium nitride crystal using an alkali metal as a flux is under study, for example. In particular, a Na flux method using metal sodium is well known. The Na flux method is a method in which metal sodium and metal gallium are mixed and heating is performed at a temperature of approximately 800° C. under nitrogen pressurization at approximately 50 atmospheres or more to synthesize gallium nitride (Patent Literature 1). There is also proposed a method in which gallium oxide and lithium nitride are reacted in a liquid gallium bath at approximately 700° C. under nitrogen pressurization at 40 atmospheres to synthesize gallium nitride (Patent Literature 2).